1. Field of the Invention
This invention relates to temperature measurement using an optical fiber and, particularly, to an optical fiber temperature sensor and a temperature measuring method.
2. Description of the Related Art
An optical fiber temperature sensor is known in which a temperature distribution along the longitudinal direction of an optical fiber is measured based on a change in transmitted light when light is inputted to an optical fiber. By placing such an optical fiber in a space such as a surface or inside of a temperature measuring object, a temperature distribution of the space can be measured. An optical fiber thermometer is also known which converts a change in transmitted light into a temperature distribution along the longitudinal direction. When the optical fiber (or optical fiber temperature sensor) is connected to the optical fiber thermometer, the temperature distribution can be measured in known method. Herein, the explanation of a principle for the temperature measurement is omitted.
However, there is a restriction as to distance resolution in the measurement results by the optical fiber thermometer. The measurement result of a point on an optical fiber is obtained as an average value of temperatures in a range of a predetermined length lying before and after the point. Namely, the length range corresponds to the distance resolution. It is impossible to measure in accurate resolution the difference of temperatures on multiple points in a region narrower than the distance resolution. Thus, the accuracy of optical fiber thermometer depends on a data sampling interval (i.e., interval in distance) in addition to the distance resolution.
FIG. 1 is a plain view showing a conventional optical fiber temperature sensor.
An optical fiber 121 is linearly placed in a space of 5 m in length. When the data sampling interval of the optical fiber thermometer is 1 m, a temperature at five points can be measured at intervals of 1 m.
JP-A-9-210809 discloses an example of an optical fiber temperature sensor.
JP-A-2002-249335 discloses an example of a holey fiber.
In order to measure a temperature (called spot temperature) in a spot region narrower than a distance resolution in a space where an optical fiber is placed by using the optical fiber with the distance resolution and an optical fiber thermometer, a method may be employed in which a predetermined length portion of an optical fiber which corresponds to the data sampling interval is concentrated in the spot region. In this case, since the predetermined length portion of the optical fiber falls within the spot region, the spot temperature can be accurately measured.
Thus, if an optical fiber concentrated part is formed to allow the predetermined length portion of the optical fiber to be concentrated in the narrow spot region, the optical fiber must have a bend. The narrower the spot region becomes, the greater the bend is (i.e., the bend radius becomes smaller). For example, when it is desired that a temperature distribution in the space as shown in FIG. 1 is measured at points as many as possible, the bend radius becomes smaller since the spot region becomes narrower as the number of the points increases.
However, in general, an optical fiber is subjected to optical transmission loss caused by the bend. As the bend radius decreases, the optical transmission loss generated at the bend portion increases. Further, when there are multiple bend portions on the entire length of an optical fiber, optical transmission losses at the respective bend portions are superposed to increase the entire length of the optical fiber. Thus, if the number of bend portions increases, the optical transmission loss increases in total.
As such, if the optical transmission loss is increased by reducing the bend radius or increasing the number of the bend portion, the temperature measurement of the optical fiber thermometer is disturbed.
For example, when a loop portion with a bend radius less than 100 mm is made, a large optical transmission loss is generated and therefore it is impossible to increase the number of the loop portion. Further, since the large optical transmission loss is generated at the loop portion, the entire length of the optical fiber needs to be reduced to suppress the total optical transmission loss. Further, due to the large optical transmission loss generated at the loop portion, temperature measurement will be disturbed and the accuracy of measurement will lower.
As such, even though it is desired to wholly and accurately measure the temperature distribution of a space, a small number of measurement points must be located in the space.